1. Field of the Invention
This invention relates generally to a powertrain for a hybrid electric vehicle (HEV), and more particularly, to controlling the actuators of a HEV when the driver requests a change from a forward to a reverse vehicle motion direction or from reverse to forward vehicle motion direction.
2. Description of the Prior Art
As illustrated in the FIG. 1, a HEV powertrain may be arranged with a first and second vehicle propulsion torque path. The first path may include an engine connected to an electric machine, such as a crank integrated starter-generator (CISG), and a multiple-speed, discrete ratio transmission connected to the electric machine, the first torque path being driveably connected to a first set of vehicle wheels. The second torque path includes an electric motor driveably connected to a second set of vehicle wheels, producing an electric rear axle drive (ERAD).
The transmission may be a powershift transmission having dual input clutches and dual layshaft gear sets or an automatic transmission having planetary gear sets with a hydrokinetic torque converter and control elements for producing forward drive in multiple gear ratios and reverse drive.
When a vehicle operator shifts a gear lever selector (i.e. PRNDL) between a forward drive position and a reverse drive position, or vice versa, the operator expects that the vehicle will slow down, stop, and then move in a direction opposite to the current direction of motion. To achieve this, in conventional vehicles, the transmission gear is disengaged from a forward gear and engaged into a reverse gear (or vice versa) if the vehicle speed is sufficiently low. But in a HEV powertrain which provides dual vehicle propulsion torque paths and is equipped with multiple torque actuators (i.e. engine, electric machines) and an automatic transmission, care has to be taken in switching from a forward to a reverse vehicle direction (or vice versa) in order to prevent unintended vehicle motion, engine stall, driveline disturbances, and potential damage to the powertrain actuators.
A need exists in the industry for a powertrain control system technique that avoids these potential problems and provides the driver's expected vehicle function.